My 55.1 3100 with a 235 and OD has gotten about 13 mpg in the 3200 miles I've driven it. A combo of in town and highway. I'd be ecstatic if I got 15 on the highway at 65 mph.
For comparison I trailered it down to Austin for the Lone Star Roundup this last weekend. I got 14.5 mpg with my Ram/Cummins tow rig. So over the 500 mile round trip, it cost me about $30 more in fuel to tow the 55 down and have 2 vehicles in town than it would to have just driven the 55 alone.
At the show a teenager asked me what kind of mileage I got from the 55. I told him that about the best I'd expect was 15. I explained that the engine design was about 50 years old, and everything the industry had learned about fuel economy had happened after its was designed.
regards,
Leon
